The invention relates generally to position sensing of hydraulic and pneumatic actuators. More particularly, it relates to sensing using laser light sources and detectors and determining the position of the actuator using time-of-flight algorithms.
Position sensing for hydraulic or pneumatic actuators typically uses an external position sensor, such as a rotary rheostat or potentiometer. Alternatively, linear rheostats or variable differential transformers are employed. These systems suffer from poor accuracy, extensive wear, and fragility in many applications, especially demanding applications such as their use on work and agricultural vehicles.
These sensors are quite susceptible to damage, and suffer from being damaged during vehicle operation, or from the extremes in temperature that work and agricultural vehicles face.
In an effort to solve these problems, new methods of measuring the position of a hydraulic or pneumatic actuator have been devised that use microwaves. These waves are transmitted from one end of the cylinder, reflect off the piston, and return to a detector. By measuring the time of flight of these waves, the location of the piston can be determined. Such an example is shown in U.S. Pat. No. 6,005,395.
The microwave transmitter suffers from high cost and difficulties in determining which of the many reflections in the cylinder is the proper one to measure.
In an alternative system, the pulse generating and timing circuit of U.S. Pat. No. 6,005,395 are used, but drive a laser light source and responds to a reflection of that beam against a laser light detector, such as that shown in co-pending U.S. patent application Ser. No. 09/750,866.
This arrangement also has drawbacks. When the piston moves toward or away from the source and detector, the reflected light follows multiple paths that, like the microwave transmitter and receiver pair, make the reflected pulses difficult to interpret. It is difficult to extract a good pulse indicative the precise time of flight of the laser beam.
What is needed is a better arrangement of laser light source and detector that provides a more precise time-of-flight measurement. It is an object of this invention to provide such an arrangement.
In accordance with a first embodiment of the invention, a fluid actuated cylindrical actuator is provided that includes a cylinder having first and second ends, an end cap fixed to the first end of the cylinder and having a rod opening, a piston disposed in the cylinder, a rod coupled to the piston and extending from inside the cylinder to outside the cylinder and passing through the rod opening, a first light guide extending from inside the cylinder to outside the cylinder and adapted to transmit at least a first beam of laser light at a first frequency from outside the cylinder to inside the cylinder and to bar the passage of the fluid, and a plurality of second light guides having first ends extending from inside the cylinder to outside the cylinder and distal second ends that are coupled to at least one light detector. Each of the second light guides may be adapted to substantially simultaneously transmit at least a first reflected portion of the beam of laser light from inside the cylinder to outside the cylinder.
The first light guide may be disposed to transmit the first beam of laser light substantially along a longitudinal axis of the cylinder such that the first beam impinges on a reflective portion of the piston over substantially an entire range of piston travel.
Each of the second light guides may be disposed on opposing sides of the first light guide such that they both receive a reflected portion of the light beam.
Each of the second light guides may be disposed substantially equidistantly from the first light guide.
The plurality of second light guides may include at least three light guides that are disposed in a semicircular arc about the first light guide.
The second ends of the plurality of second light guides may be optically coupled to a single light detector having a single electrical output generated by light carried by at least two of the plurality of second light guides.
In accordance with a second embodiment of the invention, a hydraulic actuator for an agricultural or construction vehicle is provided, the actuator including a cylinder having a substantially circular internal diameter and a longitudinal cylindrical axis, a piston having a substantially circular outer diameter and configured to be received in and hydraulically sealed against the inner diameter of the cylinder, a piston rod with a substantially circular outer rod diameter that is fixed to the piston and extends from the piston inside the cylinder, through a first end wall of the cylinder to a location outside the cylinder, wherein the first end wall is disposed to enclose and seal a first end of the cylinder and is substantially perpendicular to the longitudinal axis of the cylinder, a second end wall fixed to the cylinder and disposed to seal a second end of the cylinder substantially perpendicular to the longitudinal axis of the cylinder, the second end wall including a first optical path configured to transmit a beam of laser light through the second end wall to a reflective surface fixed to the piston and further including a plurality of second optical paths configured to transmit the reflected beam of laser light back through the end wall, a first optical fiber optically and mechanically coupled to the second end wall to transmit the beam of laser light from a remote laser light source to the first optical path and a plurality of second optical fibers optically and mechanically coupled to the second end wall to transmit the reflected beam of laser light to a remote laser light receiver.
The first optical path and the plurality of second optical paths may include at least one hermetically sealed fiber optical feed-through or connector extending through the second end wall.
The first optical fiber and the plurality of second optical fibers may be multi-modal optical fibers.
The hydraulic actuator may also include a first photo-diode configured to receive light transmitted through at least one of the plurality of second optical fibers. The actuator may include a second photo-diode configured to receive light transmitted through at least another of the plurality of second optical fibers.
The first photodiode may be disposed to receive light from at least two of the plurality of second optical fibers.
In accordance with a third embodiment of the invention, a method of determining the position of the piston of the actuator described in the previous paragraph includes the steps of generating the beam of laser light, reflecting the beam of laser light off a surface fixed to move axially with the piston, receiving a first portion of the reflected beam by a first reflected light guide and a second portion of the reflected beam by a second reflected light guide, conducting the first and second portions of the reflected beam through first and second optical fibers to at least one remotely located light detector, and calculating a first time of flight of the beam based at least upon the first and second portions of reflected light.
The method may include the step measuring a second time of flight of the beam by moving the piston to a second location in the cylinder while simultaneously increasing the optical path length of both the first and second portions of the beam an equal amount.
The step of moving the piston to the second location may include the step of filling a chamber through which the an optical chamber.
The step of generating the beam may include the step of generating the beam with a wavelength of between 500 and 1700 nanometers, or between 500 and 1400 nanometers, or between 500 and 1150 nanometers, or between 700 and 1150 and within this range (preferably between 700 and 900 or 950 and 1025 or 1030 and 1150 nanometers), 1250 and 1400, or 1450 and 1650.
The step of generating the beam may include the step of generating the beam with a wavelength in the range of 840 and 980 nanometers.
The step of generating the beam may include the step of generating a sequence of individual pulses of light, and the step of calculating the first and second times-off-light may include the step of determining the time-of-flight of at least one pulse in the sequence of individual pulses of light.